This invention relates to an improved wastewater treatment system and, in particular, to the control of an anaerobic system for treating high strength wastewater.
As evidenced in U.S. Pat. Nos.:
______________________________________ 3,640,846 3,724,542 3,817,857 3,994,780 4,043,936 4,067,801 4,100,023 4,134,830 ______________________________________
many different types of anaerobic systems have been devised for treating wastewater containing various biodegradable substrates and nutrients. All these systems operate under ambient or atmospheric conditions and are for the most part rate limited in terms of the amount of influent that can be efficiently passed through the system. The physical transport of gaseous end products through a biofilm can significantly influence the anaerobic decomposition of organic waste. Anaerobic biofilms are extremely porous materials and the rapid generation of microbubbles bubbles therein, usually of carbon dioxide and methane, can block the transport of nutrients and substrate to the organic cell sites. Transport of exoenzymes and other metabolic end products are also constrained by the microbubbles. An increase in the concentration of these materials beyond a certain level further results in product inhibition and/or the accumulation of toxic materials at the cell sites.
Molecular hydrogen (H.sub.2) is a key intermediate in controlling complex interspecies reactions occurring during the anaerobic degradation of waste in a fluid medium. Molecular hydrogen is released by one group of microorganisms into solution and used by methanogens to reduce carbon dioxide to methane. By the same token, high partial pressure of H.sub.2 in solution causes the conversion of substrate such as ethanol, propionate and bytyrate to methane to produce unfavorable free energy levels. Therefore, when operating under atmospheric conditions, the partial pressure of H.sub.2 must be maintained within a very narrow range to efficiently produce methane.
Sulfides are also produced in solution from sulfates and sulfur containing compounds that are also present in the wastewater. This, in turn, raises the toxic levels of sulfides in the biofilm to a point where the effectiveness of the anaerobic bacterial system is adversely affected.
As will be explained in greater detail below, many of the critical parameters that adversely affect the complex metabolic interactions of various species involved in anaerobic processes can be controlled or reduced by controlling the headspace pressure over the wastewater as it is being treated. By operating the system at subatmospheric conditions, gas pressures in and about the biofilms can also be relieved by reducing their solubility in the wastewater.